In an aircraft gas turbine (jet) engine, air is drawn into the front of the engine, compressed by a shaft-mounted compressor, and mixed with fuel. The mixture is combusted, and the resulting hot combustion gases are passed through a turbine mounted on the same shaft. The flow of gas turns the turbine by contacting an airfoil portion of the turbine blade that is positioned in the gas flowpath, which turns the shaft and provides power to the compressor. The hot exhaust gases flow from the back of the engine, driving it and the aircraft forward. There may additionally be a bypass fan that forces air around the center core of the engine, driven by a shaft extending from the turbine section.
The efficiency of the gas turbine engine increases with increasing temperature of the hot combustion gases, and there is therefore an incentive to operate the engine at higher combustion-gas temperatures. However, the ability to increase the combustion-gas temperature is limited by the permissible maximum operating temperatures of the components that are subjected to the highest temperatures.
One of the most demanding applications in the gas turbine engine is the turbine blades, upon which the hot combustion gases impinge, and which are also under high loads. Many techniques have been used to increase the operating temperatures of the gas turbine blades, including the use of improved metallic materials, improved manufacturing techniques, and insulating coatings. The turbine blades may be hollow, so that cooling air may be forced through the hollow turbine blades to openings from which the cooling air is expelled.
The use of ceramic gas turbine blades has been discussed and evaluated, but at this time ceramic gas turbine blades have not yet entered service. Some ceramic materials are operable to higher temperatures than are the best available metallic alloys. However, ceramic materials also tend to be of low ductilities and thence low fracture toughnesses, which may lead to premature failure of the ceramic materials in service. One possible solution is to use ceramic matrix composite (CMC) materials in which a ceramic or metallic fiber is embedded in a ceramic matrix. An example is silicon carbide fibers embedded in a silicon carbide matrix. Such CMC materials have better fracture toughnesses than do the monolithic ceramic materials.
On the other hand, the most promising of the CMC materials must be cooled, even though they are ceramics, because their maximum service temperatures in the gas turbine application are near to or less than the combustion-gas temperature. The cooling may be accomplished in essentially the same manner that conventional metallic turbine blade materials are cooled, with a flow of bleed compressor air. There have been techniques proposed to manufacture cooled turbine blades from CMC materials. The proposed techniques are complex and expensive, and have limited success.
There is therefore a need for an improved approach to the manufacture of cooled gas turbine blades from CMC materials and other types of materials, particularly low-ductility materials. The present invention fulfills this need, and further provides related advantages.